"I took your first question to be, not a speculative one, i.e., "What were Darwin's probable reasons for insisting upon gradualism, given his general principles?", but a historical one, i.e., "What were Darwin's stated reasons for insisting upon gradualism?" Therefore, I provided such quotations as I was able to find on short notice. I do not claim that they provide an exhaustive list of Darwin's reasons, and he may well elsewhere have stated the reason that you have inferred. And even if he didn't, I'm not objecting to your inference. But I'm surprised that you don't seem much interested in what he actually wrote."

It was lack of time, not interest. I posted that around midnight (with blurry eyes) and just wanted to get my point (clarification that these are principles of the Modern Synthesis, and not just Darwin, along with my own answer to the "Why?" questions) on the table before this thread could have frayed.

Since I have some time, let's look at those quotes from Darwin:

"It may be doubted whether sudden and considerable deviations of structure such as we occasionally see in our domestic productions, more especially with plants, are ever permanently propagated in a state of nature. Almost every part of every organic being is so beautifully related to its complex conditions of life that it seems as improbable that any part should have been suddenly produced perfect, as that a complex machine should have been invented by man in a perfect state."

"He who believes that some ancient form was transformed suddenly through an internal force or tendency into, for instance, one furnished with wings, will be almost compelled to assume, in opposition to all analogy, that many individuals varied simultaneously. It cannot be denied that such abrupt and great changes of structure are widely different from those which most species apparently have undergone. He will further be compelled to believe that many structures beautifully adapted to all the other parts of the same creature and to the surrounding conditions, have been suddenly produced; and of such complex and wonderful co-adaptations, he will not be able to assign a shadow of an explanation. He will be forced to admit that these great and sudden transformations have left no trace of their action on the embryo. To admit all this is, as it seems to me, to enter into the realms of miracle, and to leave those of Science."

Yes, these are very interesting and thank you very much for digging them up.

It would seem to me that Darwin thus insisted on strict gradualism because he recognized that any appearance of design ("every part of every organic being is so beautifully related to its complex conditions of life" and "many structures beautifully adapted to all the other parts of the same creature and to the surrounding conditions") could not reasonably be attributed to chance. If we remove strict gradualism, then we are removing the constant oversight of natural selection. This is just the flip side of wanting natural selection to behave as a Law. Without the "law," there is no explanation for any appearance of design.

Thus, the stubborn insistence on strict gradualism and the initial rejection of Neural Theory could be the result of wanting natural selection to mimic natural law coupled with the realization that chance without natural selection cannot mimic a designer.

I don't know whether it's safe to assume that Darwin and the proponents of the modern synthesis had exactly the same arguments in mind. Of course, they may have, but I don't know how one can be sure without comparing their writings. In any case, I have not read the writings of the founders of the modern synthesis, so I was purporting to answer only your first question.

I took your first question to be, not a speculative one, i.e., "What were Darwin's probable reasons for insisting upon gradualism, given his general principles?", but a historical one, i.e., "What were Darwin's stated reasons for insisting upon gradualism?" Therefore, I provided such quotations as I was able to find on short notice. I do not claim that they provide an exhaustive list of Darwin's reasons, and he may well elsewhere have stated the reason that you have inferred. And even if he didn't, I'm not objecting to your inference. But I'm surprised that you don't seem much interested in what he actually wrote.

In any case, while I am here, I wish to issue a correction to my previous reply to you. In my post, I wrote:

"3. Darwin noted the interaction of complex parts and systems in an organism, and reasoned that for a great leap to occur, a completely new organization would have to pop up, and that this would be wildly unlikely. (Chs. II, VII)

"Regarding reason 3, note that Darwin was anticipating the "tornado assembling a 747 in a junkyard" objection to evolution, and saying that he agreed with the objection: evolution couldn't possibly work in that way. (Note also that reason 3 employs essentially Paleyesque-ID type reasoning, including the analogy to machines, so that if the core arguments of Paley and ID are to be banned from the schools as "religious", one of Darwin's own arguments against gradualism must also be banned from the schools as "religious".)"

In the last sentence, I should have written "one of Darwin's own arguments FOR gradualism" or "one of Darwin's own arguments AGAINST saltationism".

Thus, I don't think observations about blending theory or Lamarkianism apply. Those might partly explain Darwin's thinking, but they fail to explain the Modern Synthesis - its commitment to gradualism and its original rejection of Neutral Theory.

My guess is that Darwin, like so many other great scientists, was greatly influenced by Newton's success in finding a small number of simple laws that explained just about everything. In other words, Darwin and the architects of the Modern Synthesis, sought a simple, elegant explanation for all of evolution. And in this Explanation, natural selection was supposed to be ubiquitous because then natural selection would exist almost as a Law.

So Darwin would insist on strict gradualism because it meant that Natural Selection was effectively omnipresent, always scrutinizing every bit of variation that popped into existence. And we can change Koonin's observation, "the Modern Synthesis, in its 'hardened' form, effectively, rejected drift as an important evolutionary force, and adhered to a purely adaptationist model of evolution" to ""the Modern Synthesis, in its 'hardened' form, effectively, rejected drift as an important evolutionary force *because it* adhered to a purely adaptationist model of evolution." The "purely adaptationist model of evolution" comes from viewing natural selection as omnipresent and the existence of drift as an important evolutionary force denies such omnipresence.

The advantage to my explanation is that it answers both "why?" questions with great parsimony.